This same measurement applies to the spark plugs, the points of which must provide a gap of not more than one sixty-fourth of an inch.
TROUBLES
If there is an abrupt failure of ignition, it is probable that the primary is short-circuited, and this can only occur on the wire that leads to the short-circuiting switch, for the other parts of the primary circuit are inclosed and protected against injury.
In case of the missing of one cylinder only, the trouble will usually be found in the plug, which may be short-circuited by a carbon deposit, or because the intense heat of the spark has fused the metal, resulting in the formation of a globule of metal between the spark points. Too great a distance between the points, more than one sixty-fourth of an inch, will prevent the passing of the spark, which will then be seen in the safety spark gap.
A miss in different cylinders may be due to defective insulation of the wires, but if these are in good condition the fault may be looked for in the magneto. The place where trouble is usually experienced is in the interruptor, which may become dirty, or the contacts may be worn down or loosened from the vibration. Fouled contacts in the distributor may also lead to a miss-fire, and this will be corrected by wiping the parts with gasoline. If all of these parts have been examined and found correct, it is inadvisable to examine further into the magneto, for inexperienced handling will be likely to injure the delicate parts. It is better to place it in the hands of a man whose shop is equipped for the work.
FOUR-SPARK MAGNETO
All of the magnetos described have been of the type in which the armature revolves, and two ignitions are secured per revolution. These are known as two-spark magnetos, or crank-shaft speed magnetos, from the fact that for four cylinder engines the magneto runs at the speed of the crank shaft. This is in distinction to the Bosch four-spark, or cam-shaft speed, magnetos, in which the armature as well as the field is stationary. This magneto for its simplicity, freedom from trouble, and workmanship, has few equals for the high-tension ignition of an internal combustion engine.
Fig. 13.—Diagrams showing Position of Shield Revolving about Armature.
The armature in this magneto is of the usual type, and is stationary, with the armature neck in a vertical position. Around the armature, and between it and the pole pieces, revolves a soft iron shield in two sections, these being the length of the armature and the same width as the heads. As it revolves, it forms bridges between the pole pieces and the core of the armature, the lines of force flowing through it as well as through the armature. The diagrams of Fig. 13 show the positions of the shield as it revolves about the armature. In the first position, one segment of the shield forms a bridge between the pole piece and the upper armature head, while the other segment is bridging the space between the lower head and the other pole piece. When in this position, the lines of force flow through the neck of the armature and magnetize it so that it sets up its own field. When the shield revolves so that it covers the heads, the lines of force abandon the neck and flow between the pole pieces by the segments, the field established by the neck dying out. In the third position, the lines of force again flow through the neck, while in the fourth position, with the shield completing a half revolution, the lines of force again pass directly across. In this half revolution there are therefore two periods when a magnetic field forms around the armature neck and dies out, which will result in the induction of two currents in the winding of the armature. If the revolution is continued it will be seen that the same conditions are repeated, the positions of the two segments of the shield being reversed, and that one revolution of the shield about the armature will produce four waves of current in the winding. The magneto may therefore be driven at cam shaft speed for a four cylinder engine, and in addition has the advantage of the wire windings being stationary. The action of the magneto is the same as that of the two-spark type already described, but the construction is simplified because the slow speed of the shield will permit the secondary distributor to be attached directly to the armature shaft, the secondary shaft being done away with.